A typical process flow for manufacturing replacement metal gate (RMG) transistors may include removing a temporary gate (sometimes referred to as a dummy gate), leaving a trench where various layers of material are deposited to form the RMG. For example, a dielectric layer may be deposited into the trench, followed by a first metal layer, a second metal layer, and a conductor layer. As will be appreciated, when each layer is deposited into the trench, material may be deposited onto the bottom of the trench as well as the sidewalls. The material build-up on the sidewalls, however, is not required for the RMG transistor to function properly. In some cases, the material build-up on the sidewalls may actually reduce the performance of the transistor. For example, some high-k dielectric build-up on the sidewalls of the trench may increase the parasitic capacitance within the RMG transistor and cause cross-talking with adjacent contacts.
As device structures and sizes are scaled down, the width of transistor gates also decreases. As such, the width of the trench described above for an RMG transistor also decreases. With each layer of material being deposited into the trench, the material build-up on the sidewalls of the trench further decreases the trench opening for subsequent layer depositions. Furthermore, each layer of material requires a minimum thickness to properly function. Accordingly, there are theoretical limits to the minimum gate width, the number of layers and each layers respective minimum thickness.
Additionally, as will be appreciated, deposition may be affected by the aspect ratio of the trench. A trenches aspect ratio is often represented as the ratio of the trench height to trench width. Deposition may be non-uniform at higher aspect ratios, which may manifest as thicker deposition higher on the sidewalls and thinner deposition lower on the sidewalls and at the bottom of the trench. With each successive deposition of a layer in the RMG, the aspect ratio of the trench will increase, possibly further exacerbating the non-uniformity of the deposition and further decreasing the trench width.
As such, there is a need to remove at least some portion of the material deposited on the sidewalls of the trench, in order to improve the transistor device performance, reduce the aspect ratio of the trench, improve quality and uniformity of depositions, and to allow devices with smaller gate widths to be manufactured.